Use the DTSHD fourcc instead of the hacked "dtshd" variable.

The DTSHD -> DTS fallback is now handled in aout core, the same code path will
be used for EAC3 -> AC3 fallback. Therefore, aout modules don't have to handle
the DTSHD -> DTS fallback anymore. If a module can't handle DTSHD, it should
return an error and the DTS fourcc will be tried right after.